Quadrature amplitude modulation (QAM) is an intermediate frequency (IF) modulation scheme in which a QAM signal is produced by amplitude modulating two baseband signals, generated independently of each other, with two quadrature carriers, respectively, and adding the resulting signals. The QAM modulation is used to modulate a digital information into a convenient frequency band. This may be to match the spectral band occupied by a signal to the passband of a transmission line, to allow frequency division multiplexing of signals, or to enable signals to be radiated by smaller antennas. QAM has been adopted by the Digital Video Broadcasting (DVB) and Digital Audio Visual Council (DAVIC) and the Multimedia Cable Network System (MCNS) standardization bodies for the transmission of digital TV signals over Coaxial, Hybrid Fiber Coaxial (HFC), and Microwave Multi-port Distribution Wireless Systems (MMDS) TV networks.
The QAM modulation scheme exists with a variable number of levels (4, 16, 32, 64, 128, 256, 512, 1024) which provide 2, 4, 5, 6, 7, 8, 9, and 10 Mbit/s/MHz. This offers up to about 42 Mbit/s (QAM-256) over an American 6 MHz CATV channel, and 56 Mbit/s over an 8 MHz European CATV channel. This represents the equivalent of 10 PAL or SECAM TV channels transmitted over the equivalent bandwidth of a single analog TV program, and approximately 2 to 3 High Definition Television (HDTV) programs. Audio and video streams are digitally encoded and mapped into MPEG2 transport stream packets, consisting of 188 bytes.
The bit stream is decomposed into n bits packets. Each packet is mapped into a QAM symbol represented by two components I and Q, (e.g., n=4 bits are mapped into one 16-QAM symbol, n=8 bits are mapped into one 256-QAM symbol). The I and Q components are filtered and modulated using a sine and a cosine wave (carrier) leading to a unique Radio Frequency (RF) spectrum. The I and Q components are usually represented as a constellation which represents the possible discrete values taken over in-phase and quadrature coordinates. The transmitted signal s(t) is given by: EQU s(t)=I cos(2.pi.f.sub.0 t)-Q sin(2.pi.f.sub.0 t),
where f.sub.0, is the center frequency of the RF signal. I and Q components are usually filtered waveforms using raised cosine filtering at the transmitter and the receiver. Thus, the resulting RF spectrum is centered around f.sub.0 and has a bandwidth of R(1+.alpha.), where R is the symbol transmission rate and .alpha. is the roll-off factor of the raised cosine filter. The symbol transmission rate is 1/n.sup.th of the transmission bit rate, since n bits are mapped to one QAM symbol per time unit 1/R.
In order to recover the baseband signals from the modulated carrier, a demodulator is used at the receiving end of the transmission line. The receiver must control the gain of the input amplifier that receives the signal, recover the symbol frequency of the signal, and recover the carrier frequency of the RF signal. After these main functions, a point is received in the I/Q constellation which is the sum of the transmitted QAM symbol and noise that was added over the transmission. The receiver then carries out .alpha. threshold decision based on lines situated at half the distance between QAM symbols in order to decide on the most probable sent QAM symbol. From this symbol, the bits are unmapped using the same mapping as in the modulator. Usually, the bits then go through a forward error decoder which corrects possible erroneous decisions on the actual transmitted QAM symbol. The forward error decoder usually contains a de-interleaver whose role is to spread out errors that could have happened in bursts and would have otherwise have been more difficult to correct.
Generally, in transmitting a modulated signal, two impairments are encountered, phase noise and additive noise. Phase noise is generated by the various mixers and local oscillators in the modulator and the demodulator. The sidebands of the phase noise signal are coherent, which means that the upper frequency sidebands have a definite phase relationship to the lower frequency sidebands. Additive noise, also referred to as additive gaussian white noise, is random noise that has a frequency spectrum that is continuous and uniform over a specified frequency band. It is often very difficult to evaluate the amount of phase noise or additive noise for which the demodulator should compensate. In order to compensate for phase noise, the carrier loop bandwidth has to be increased. However, this causes the signal degradation caused by the additive noise to increase. In order to compensate for the additive noise, the carrier loop bandwidth should be decreased, but this causes the effect of increasing the phase noise degradation of the signal.
In the prior art, several attempts have been made to compensate for or to eliminate phase noise and/or additive noise. U.S. Pat. No. 5,315,618 to Yoshida discloses a method and apparatus for cancelling periodic carrier phase jitter. In the Yoshida invention, if a demodulated complex baseband signal is deviated in phase from a QAM signal point due to phase jitter, the phase error is detected, and a replica of the phase jitter is calculated and applied to impart phase rotation for cancelling out the phase jitter that is contained in the complex baseband signal. U.S. Pat. No. 4,675,613 to Naegeli et al. discloses a circuit in a synchronous detector system that is provided to minimize and compensate for the errors induced by phase modulation and additive noise in the system. In one embodiment, a first-order correction of such errors is achieved by equipping the synchronous detector system with a phase lock loop having a constant loop filter noise bandwidth to reduce the phase noise and an RMS detector for first order correction of the additive noise. The resolution filter passing the signal to the RMS detector is made to have a noise bandwidth identical to the loop noise bandwidth. U.S. Pat. Nos. RE 31,351 and 4,213,095 to Falconer discloses, respectively, a feedback nonlinear equalization of modulated data signals and a feedforward nonlinear equalization of modulated data signals. In the '351 patent, a receiver for a QAM signal impaired by linear and non-linear distortion, phase jitter and additive noise includes circuitry which compensates for these impairments. In particular, the receiver includes a processor which subtracts a feedback nonlinear signal from each sample of the received signal, either prior to or subsequent to demodulation, providing compensation for non-linear intersymbol interference. In the '095 patent, a feedforward non-linear signal is added to each sample of a linearly equalized received signal to provide compensation for nonlinear intersymbol interference. In each of the patents, the feedback/feedforward nonlinear signal is comprises of a weighted sum of products of individual ones of the samples and their complex conjugates.
It is an object of the present invention to provide a QAM type demodulator that provides a joint estimation of the phase noise and the additive noise, while limiting the mutual effect induced by one of the estimations on the other of the estimations.